# CBSE class XII Physics Optical instruments 2008

Q. 1. A telescope has an objective of focal length 50 cm and eyepiece of focal length 5 cm .The least distance of distinct vision is 25cm.The telescope is focused for distinct vision on a scale 200 cm away from the object. Calculate (a) the separation between the objective and eyepiece and (b) the magnification produced. (70.83 cm, -2)

Q. 2. A telescope objective of focal length 1 m forms a real image of the moon 0.92 cm in diameter. Calculate the diameter of the moon taking its mean distance from the earth to be 38x104 km. (3.5 x106 m)

Q. 3. An angular magnification of 30X is desired using an objective of focal length 1.25 cm and an eyepiece of focal length 5 cm. How will you set up the compound microscope? (L=11.67cm.)

Q. 4. A compound microscope is used to enlarge an object kept at a distance of 0.30 m from its objective, which consists of several convex lenses and has focal length 0.02 m. If a lens of focal length 0.1 m is removed from the objective, find out the distance by which the eyepiece of the microscope must be moved to refocus the image? (9cm.)

Q. 5. An amateur astronomer wishes to estimate roughly the size of the sun using his crude telescope consisting of an objective lens of focal length 200 cm and eyepiece of focal length 10 cm. By adjusting the distance of the eyepiece from the objective, he obtains an image of the sun on a screen 40 cm. behind the eyepiece. The diameter of the sun’s image is measured to be 6 cm. What is his estimate of the sun’s size, given that average earth – sun distance is 1.5x1011 m. (1.5 x 109 m)

Q. 6. A 35 mm slide with a 24 mm x 36 mm picture is projected on a screen placed 12 m from the slide. The image of the slide picture on the screen measures1.0mx1.5 m. Determine the location of the projection lens and its focal length? (28.1cm, 27.5 cm)

Q. 7. An eyepiece of a telescope consists of two plano convex lenses L1 and L2 each of focal length f separated by a distance of 2ƒ/3. Where should L1 be placed relative to focus of the objective lens of telescope, so that the final image through L2 is seen at infinity? (ƒ/4)

Q. 8. A telescope has a objective of diameter 60 cm. The focal lengths of the objective and eyepiece are 2 m and 1.0 cm respectively. The telescope is directed to view two distant point sources of light (e.g. two stars of a binary). The sources are roughly at the same distance ( 104 light years) along the line of sight, but separated transverse to the line of sight by a distance of 1010 m. Will the telescope resolve the two objects? (Take, λ =6x10-7 m for mean yellow color.) (No)

Q. 9. The separation between the eyepiece (of focal length 0.3 m) and objective (of focal length 0.4 m) of a microscope is 0.2 m. The eyepiece and the objective are to be interchanged such that the angular magnification of the instrument remains the same. What is the new separation between the lenses? Hint: use, m = (0.3m)

Q. 10. The image of the moon is focused by a converging lens of focal length 50 cm on a plane screen. The image is seen by an unaided eye from a distance of 25 cm. Find the angular magnification achieved due to the converging lens? (- 2)

Q. 11. The objective of telescope A has a diameter 3 times that of the objective of telescope B. How much greater amount of light is gathered by A compared to B? Show that range of A is three times the range of B? [Hint: Intensity α ] (9 times)

Q. 12. The diameter of sun is several hundred times bigger than the moon, still at the time of solar eclipse, the moon covers the entire sun. How?

Q. 13. How does magnifying power change with change in length of tube of a given telescope and microscope each?

Q. 14. Distinguish between linear magnification and angular magnification. When are these two magnifications equal? 15. When viewing through a compound microscope, our eye should be positioned not on the eyepiece, but a short distance away from it. Why? How much should be that short distance between the eye and the eyepiece?